Abstract

Porous Vycor glass with nano-sized pores is transparent in the visible region and is often used in colorimetric chemical sensing, when it is impregnated with selectively reacting reagents. However, it has some disadvantages in its use, since changes in the humidity of ambient air strongly affect the transmission. In this work, we analyzed the transparency change during the drying process to correlate the turbidity of the glass with the amount of water in it. The transparency change in the visible region takes place for the duration of the drying and is found to be dependent on the inverse 4th power of the wavelength (1/λ4), which implies that Rayleigh-type scattering takes place during the drying process. Based on the above observation, it is shown that the transitory white turbidity of nanoporous glasses during the drying process can be interpreted consistently and quantitatively analyzed by a simple Rayleigh scattering mechanism.

Changes in the common logarithm of the transmission (which is proportional to the turbidity τ) as a function of the inverse 4th power of wavelength (1/λ4). The slope initially decreases gradually, then becomes steeper, and recovers again gradually. The pore filling fraction (f) with water is estimated from the absorbance peak at around a wavelength of 1900 nm, normalized by the initial maximum value measured immediately after the removal from immersion.

Time dependence of the slope β of turbidity (τ) versus 1/λ4 plots and of the absorbance peak (α1900) at around a wavelength of 1900 nm. The slope changes remarkably as the time passes. Initially a small value of 1.01×10−6μm3, it then increases to the maximum value of 8.81×10−6μm3, and then decreases and saturates to the value of 8.11×10−7μm3, which is a little smaller than the starting value.

Slope β in the 350–800 nm range as a function of the pore filling fraction f extracted from the peak absorbance at around 1900 nm. The slope reaches its peak value of about 8.81×10−6μm3 at about f=0.6.

Volumetric adsorption isotherm for nitrogen on porous Vycor glass. The horizontal axis shows the ratio of the pressure to the saturated pressure of the nitrogen gas at 77 K. The specific surface area as measured from the BET plot from 0.02 to 0.5P/P0 is 207m2/g, which coincides with the specifications for the glass [11].

Scatterer’s effective radius (rsca) and number density (N) as a function of the pore filling fraction. The effective radius of Rayleigh scatterers is initially about 6.9 nm, then becomes the maximum at about 9.4 nm, and finally reduces to its minimum of about 3.2 nm. Correspondingly, the number density of scatterers starts with a value of 2.20×1017cm−3, then becomes the minimum of 8.67×1016cm−3, and finally approaches the maximum of 2.16×1018cm−3.